1. Field of the Invention
The present invention relates to a discharge tube and more particularly to a discharge tube suitably applied to a series gap ignition apparatus in automotive engines.
2. Description of the Prior Art
An ignition apparatus C for automotive engines as shown in FIG. 3 has come to be known in recent years. In this ignition apparatus (as described in the Japanese Patent Publication No. Showa 51-32180), what is generally called a series gap S is provided in series with an ignition plug 1 to prevent the ignition plug 1 from smoldering due to adhering carbons and thereby keep the ignition timing constant. It is being thought of to form the series gap S with a so-called discharge tube 2, which is sealed with an inert gas and applied with a voltage between electrodes at each end of the tube to cause a discharge.
FIG. 4 shows a above-mentioned conventional discharge tube 2, which is installed in a plug cap (not shown) that is removably mounted to the ignition plug 1. The discharge tube 2 has a casing 3 as a hollow cylindrical insulating tube that is formed of ceramics. The casing 3 consists of a body portion 3a which at one end is closed inwardly and at the other end open, and a cover portion 3b that fits airtightly onto the open end of the body portion 3a. At each end of the casing 3 are formed openings 4, into which a pair of Rogowskii type perforated electrodes 6 or so-called flanged electrodes are fitted and projected toward each other, with a specified distance, i.e., a series gap S provided inside the casing 3 between the facing ends of the electrodes. The flanged electrode 6 has its base portion 6a attached to an electrode cap 5 that covers the opening 4. One of the electrode caps 5 is attached with a sealing pipe 7 through which an inert gas such as argon is loaded and sealed.
In the conventional discharge tube 2 mentioned above, the pair of electrodes 6 hermetically fitted to the casing 3 are equal in shape so that the discharge tube itself has no directivity. Thus when a voltage is applied across the electrodes 6, with one electrode taken as an anode and the other as a cathode, a discharge occurs between the tips 6b or inner ends of the electrodes 6, i.e., in the series gap S. The discharge voltage in the series gap S is kept at a relatively high level to apply the high voltage after discharge to the electrodes of the ignition plug 1 so that an ignition voltage required by the ignition apparatus C can be produced without being much affected by carbons adhering to the ignition plug 1.
In the above conventional discharge tube 2, however, since the pair of flanged electrodes 6 are projected into the casing 3 toward each other with a specified gap therebetween, the inner space of the casing 3 is larger than the discharge space between the electrode tips 6b. This in turn makes large the discharge tube 2 and therefore the plug cap that contains the discharge tube 2, making it impossible to reduce the size of the ignition apparatus.
Another problem is that while the discharge is considered to occur theoretically between the electrode tips 6b whose distance is the shortest, there are rare cases where the discharge occurs not between the electrode tips 6b but between the base portions 6a. Such an unstable discharge passage in the discharge tube 2 results in an unstable discharge voltage, which in turn gives rise to a problem that the ignition apparatus C may in some cases not be able to get the required ignition voltage.